The use of high power lasers, for example, those with pulse energy densities (fluence) above 20 mJ/cm2, with pulse lengths in the low nanometer range, can degrade the optics used in laser lithography systems. T. M. Stephen et al., in their article “Degradation of Vacuum Exposed SiO2 Laser Windows” SPIE Vol. 1848, pp. 106-110 (1992), report on the surface degradation of fused silica in Ar-ion laser. More recently, it has been noticed that there is optical window surface degradation in high peak and average power 193 nm excimer lasers using window materials made from substances other than silica. There is also evidence that such degradation will be more severe when existing optical materials are used in 157 nm laser systems. While some solutions such as using MgF2 as the window or lens material for existing 193 nm laser systems have been proposed, it is believed that such materials will also experience surface degradation with time, leading to the requirement that the expensive windows be periodically replaced. It is further believed that the problem with window degradation will be exacerbated with the advent of laser systems operating at wavelengths below 193 nm. In addition, the use of MgF2 as a window material, while it might be successful from a mechanical viewpoint, presents a problem of color center formation that is detrimental to transmission performance of the laser beam.
Excimer lasers are the illumination sources of choice for the microlithographic industry. While ionic materials as such as crystals MgF2, BaF2 and CaF2 are the materials of choice for excimer optical components due to their ultraviolet transparencies and to their large band gap energies, the preferred material is CaF2. However, crystals of CaF2 and the optical elements made from CaF2 are difficult to optically polish. Furthermore, polished but uncoated surfaces of CaF2 are susceptible to degradation when exposed to powerful excimer lasers operating in the deep ultraviolet (“DUV”) range of less than 250 nm, for example at 248 and 193 nm. For lasers operating at 193 nm, 2 KHz or 4 KHz, with pulse energy densities of 20-40 mJ/cm2, the surfaces or the optical elements made from these ionic materials are known to fail after only a few million laser pulses. The cause of the damage is thought to be fluorine depletion in the top surface layers of the polished surface. See Wang et al., “Color center formation on CaF2 (111) surface investigated by using low-energy-plasma-ion surfacing”, Optical Society of America 2004, [2004_OSA_OF&T] and Wang et al., “Surface assessment of CaF2 deep-ultraviolet and vacuum-ultraviolet optical components by the quasi-Brewster angle technique,” Applied Optics, Vol. 45, No. 22 (August 2006), pages 5621-5628. U.S. Pat. No. 6,466,365 (the '365 patent) describes a method of protecting metal fluoride surfaces, such as CaF2, from degradation by use of a vacuum deposition, of a silicon oxyfluoride coating/material. While for the moment this is a reasonable solution, the microlithographic industry constantly demands more performance from excimer sources, and consequently from optical components used in connection with Excimer laser based systems. Therefore, in view of the expected increased industry demands for improved laser performance, it is desirable to find a solution to the optical element degradation problem that will either eliminate the problem or will greatly extend the durability, and consequently the length of time that existing and future optical components can be used.